This invention concerns weighing apparatus for batches of flowable material, for example powders or granular materials, and in particular apparatus for producing batches containing predetermined proportions of various different materials by weight.
Amongst other applications, the apparatus may be used to produce pre-packaged batches of chemical additives, to be mixed into the main components of a plastics melt or rubber compound during manufacture. Such additives are known as xe2x80x9csmall chemicalsxe2x80x9d. They are often supplied to polymer producers in thin plastics bags containing as many as sixteen different chemical ingredients, in the correct proportions by weight, for addition to a standard quantity of the main constituents of the polymer mix. The bags are either emptied into the main mix, or are simply dropped in whole, to be ruptured and mixed in during subsequent processing.
The small chemicals are usually weighed out manually. Each is delivered from a sack tip into a storage bin. Weighing apparatus mounted on a trolley is wheeled from bin to bin and at each a quantity of chemical is removed by hand, weighed out to the correct amount, and added to the mix for bagging. This process is labour intensive, relatively slow, requires skill, is prone to human error and may also carry health and safety implications. Apparatus is known for automatically weighing out correct quantities of the small chemicals, in which the various ingredients are fed by conveyors from bulk storage to a batch weigher and an associated bagging station. The chemicals are fed and weighed in succession. Because the number of ingredients is large, the bagging cycle times are relatively slow.
Also, if vibratory or screw feeders are used (as are particularly convenient for dispensing powders or granular materials), to avoid blockages it is best to keep the lengths of the individual conveyors as short as possible. As a rule of thumb, desirably, no conveyor should exceed 3 metres in length. Devising a compact apparatus layout that can be used to weigh and combine in batches a large number of ingredients, and which keeps the conveyor lengths acceptably short, is difficult.
In accordance with the present invention, apparatus for combining different flowable materials into batches in predetermined proportions by weight comprises a plurality of bulk material supply stations, a plurality of weighing hoppers, and a plurality of conveyors extending between the bulk material supply stations and the weighing hoppers, wherein the weighing hoppers have an upper end for receiving material from the conveyors, and a discharge end, the weighing hoppers being grouped together in close proximity, outwardly arranged side walls of the group of weighing hoppers sloping inwardly and downwardly, so that material emerging from the discharge ends is directed along convergent discharge paths, the weighing hoppers each comprising a hinged discharge door over which in the open position the material flows, thereby forming part of the convergent discharge path.
The discharge ends may direct the weighed material into a common discharge funnel, which in turn may direct the material into bagging apparatus.
For a particularly compact arrangement, the weighing hopper upper ends are preferably disposed along a substantially circular pitch line in a substantially horizontal plane. Adjacent sides of the weighing hopper upper ends preferably conform to one another to make a space-filling array. The weighing hoppers may define a central space for dust extraction.
The weighing hoppers may each be mounted upon respective off-centre load cells. There are preferably at least five weighing hoppers, for example eight arranged in an octagonal formation. Although larger numbers are possible, preferably no more than two of the conveyors discharge into a given weighing hopper. The use of multiple weighing hoppers enables weighing operations to be carried out for different ingredients simultaneously, so minimising cycle times. The use of more than one conveyor to each weighing hopper correspondingly multiplies the number of ingredients that can be weighed out in each batch. The conveyors may be screw, vibratory, or of any other suitable kind, and are desirably less than 3 metres in length. When very accurate proportioning is required, each ingredient supplied is fed and weighed in each bagging cycle. To decrease average bagging cycle times, by eliminating the need for successive ingredient feeding in a given cycle, as well as eliminating the time taken for weight measurements to settle and for weighing hoppers to discharge, weight measurements need not be taken during every bagging cycle. During each bagging cycle, the conveyors are operated for timed periods calculated on the basis of the previous weight measurements, so that these converge to target values.
The bulk material supply stations are conveniently positioned above and outwardly of the weighing hoppers. Adjacent pairs of bulk material supply stations may be connected to a given weighing hopper by the conveyors. These pairs of supply stations may be arranged around the sides of a rectangle surrounding the weighing hoppers.